Things We Think Are True

This blog post about sleep and depression started like this:

All of us have things we believe in even though we don't really have any evidence for them.

Isn't that the truth? For example, I'm pretty sure large amounts of coffee probably aren't good for us and in fifty years we'll be saying, "Why didn't we see that that wasn't good for us?!?!" though I've never read any scientific study that supports that theory. What about you all? What is something you believe, but have no proof of its validity?